Positron emission tomography (PET) includes injection of a radiopharmaceutical into a patient, and scanning the patient. The decay of the radiopharmaceutical produces positrons. When a positron meets an electron, an annihilation event occurs, emitting two gamma rays in opposite directions. The scanner has multiple rings of scintillators, which capture the annihilation photons emitted from a patient or subject. An array of detectors detect the photons and outputs signals indicating the energy and the time at which the detector detects the photons.
Annihilation events can be categorized as “prompts” or “randoms”. If the leading edges of the two time marks both occur within a “coincidence window” (e.g., typically about 4.0 to 4.2 nanoseconds, depending on the size of the scanner and detector time resolution), the two detections are considered to be produced by a single annihilation event, and indicate a prompt event. Prompt events also include random events that are not generated from one annihilation point. In order to measure the random events, one can implement a delay circuitry. In this circuitry, signals corresponding to one of the annihilation events are delayed much longer than the coincidence window so that any coincidence detection in this part of the electronics will be accidental. These detections will be considered as random events. “True” coincidence events are the difference between prompt events and random events.
A PET system includes a coincidence circuit for determining whether a pair of received signals indicate a prompt coincidence event or a random event.